Various systems for three dimensional modelling have been proposed. There is described in U.S. Pat. No. 4,575,330 to Hull, apparatus for production of three-dimensional objects by stereolithography. The system described therein is intended to produce a three-dimensional object from a fluid medium capable of solidification when subjected to prescribed synergistic stimulation and comprises apparatus for drawing upon and forming successive cross-sectional laminae of the object at a two-dimensional interface and apparatus for moving the cross-sections as they are formed and building up the object in step wise fashion, whereby a three-dimensional object is extracted from a substantially two-dimensional surface.
An earlier publication by Hideo Kodama entitled "Automatic method for fabricating a three-dimensional plastic model with photo-hardening polymer", Rev. Sci Instrum. 52 (11) November, 1981, pp. 1770-1773 describes many of the features appearing in the Hull patent as well as additional features.
An article by Alan J. Herbert entitled "Solid Object Generation" in Journal of Appled Photographic Engineering 8: 185-188 (1982) describes the design of apparatus for producing a replica of a solid object using a photopolymer.
FIG. 5 of the Hull Patent and FIGS. 1a and 1b of the Kodama article illustrate layer by layer buildup of a model through radiation applied to a solidifiable liquid through a mask using a "contact print" technique. Accordingly, the pattern mask for each layer must be in a 1:1 scale relationship with the object to be generated and must be located extremely close to it.
A number of difficulties are involved in the use of a contact print technique due to the required 1:1 scale. If a complex object having a typical size of up to 10 inches on each side is contemplated and resolution of 100 microns is desired, approximately 2500 masks will be required, covering an area of over 150 sq. meters. An extremely fast mechanism for moving and positioning the masks and the use of non-standard film of a given size for a given scale output would be required.
The required proximity of the mask to the object in contact print exposure is not believed to be desirable in an industrial environment because of anticipated contact between the mask and the solidifiable liquid due to vibrations in the liquid during positioning and movement of the masks and due to spurious impacts.
Neither Kodama nor Hull provides apparatus for accurate positioning of the mask and accurate registration of masks for different layers. The positioning error must not exceed the desired resolution, typically 100 microns.
Both Kodama and the Hull patent employ an arrangement whereby the object is built up onto a base which lies in a container of solidifiable liquid and moves with respect thereto. Such an arrangement involves placing a base displacement mechanism in the container and in contact with the solidifiable liquid. Due to the high viscosity and glue-like nature of such liquids, it is believed to be impractical to operate such a system, particularly when it is desired to change materials in order to vary the mechanical properties or color of the object being generated.
Furthermore, should excessive radiation be applied to the liquid, the entire volume might solidify, thus encasing the support mechanism therein.
Another difficulty with the apparatus for support of the model in the prior art exemplified by Kodama and Hull lies in maintenance of the stability of the solidifiable liquid. Both Hull and Kodama move the model in the liquid, causing disturbances in the liquid and thus requiring time for the liquid to settle after each such displacement. Hull describes movement of the light source but not as a substitute for the displacement of the object relative to the liquid.
The technique of supplying solidifiable liquid into the container in the course of building up the model is not specifically described in either Kodama or Hull. Herbert shows at FIG. 3, a tap which releases solidifiable liquid well above the liquid surface.
Definition of the bottom limit of solidification for a given layer is achieved in the Hull and Kodoma references by precise control of irradiation energy levels. Due to the fact that energy intensity decreases exponentially with depth within the liquid, this technique does not provide a sharp definition in layer thickness, as noted by Hull on pages 9 and 10, referring to FIG. 4. Hull suggests solving the problem of bottom limit definition by using an upwardly facing radiation technique which is not applicable to many geometrical configurations.
The prior art exemplified by the Kodama and Hull references does not provide teaching of how to model various geometries which involve difficulties, for example a closed internal cavity, such as a hollow ball, isolated parts, such as a linked chain, and vertically concave shapes, such as a simple water tap. The identification of situations which require the generation of support structures and the automatic generation of such structures are not suggested or obvious from the prior art.
An additional difficulty involved in prior art modelling techniques of the type exemplified by the Kodama and Hull references, but which is not explicitly considered by either is shrinkage of the solidifiable liquid during solidification. Normal shrinkage for most of the available monomers employed in the prior art is about 8% in volume and 2% in each linear dimension. This shrinkage can affect the dimensional accuracy of the three dimensional model in the following principal ways: two-dimensional linear scale changes, two-dimensional non-linear distortions due to internal stresses with each individual layer as it solidifies and three-dimensional distortions due to stresses arising from stresses in the overall model during a final curing step.
The Hull technique suggests the use of direct laser writing in a vector mode, which requires extreme uniform writing speed in order to maintain a constant energy level and produce a uniform layer thickness.